1. Field of the Invention
This invention relates to the field of charge transfer device memories having permanent non-volatile storage capabilities.
2. Description of Prior Art
Charge transfer device memories utilizing MNOS storage are known in the prior art. These memories are of two types, a first type provides permanent storage locations over the charge transfer channel and a second type provides permanent storage locations which are displaced laterally from the charge transfer channel. Memories of the first type are simpler to operate than those of the second type, but are less efficient because of an excessive number of fast states at the surface of the charge transfer channel. The excessive fast states result from the overlying nitride layer which prevents the excessive fast states from being annealed out. Memories of the second type provide more efficient charge transfer action than memories of the first type. The more efficient charge transfer action results from annealing of the charge transfer channel, is effective in memories of the second type because their nitride layer does not overlie the propagation portion of the channel.
The operation of the second type memory is more complicated than that of the first type memory because an electrode and a control voltage must be provided to control cummunication between the propagation portion of the charge transfer channel and portion of the charge transfer medium underlying the permanent storage locations in order to prevent a loss of transfer efficiency during charge transfer as a result of undesired spreading of charge into the portion of the charge transfer medium underlying the permanent storage sites.